One of the most common intersex conditions is Androgen Insensitivity Syndrome (AIS). People who have Androgen Insensitivity Syndrome have XY chromosomes (usual male chromosomes), but their bodies do not have the same response to testosterone as most peoples' do. They often appear to be female based on external characteristics but do not have the usual female reproductive organs. Many people who have Androgen Insensitivity Syndrome live their whole lives without knowing it. There have even been Olympic athletes with AIS.
For more information about AIS in athletes:
http://www.hhmi.org/biointeractive/gendertest/gendertest.html
